Process for making diphenyl



Patented Sept. 5, 1933 UNITED STATES PATENT GFFICE to The Dow Chemical Company, Midland, Mich., a corporation of Michigan Application February 9,

11 Claims.

The present invention is concerned with a process for making diphenyl by heating benzene under conditions such that hydrogen is split therefrom and the phenyl residues unite to form the desired product,'i. e. diphenyl.

It has long been known that diphenyl can be prepared by pyrolyzing benzene, the temperature of such pyrolysis being variously reported in the literature to be above approximately 650 C. The yield of diphenyl, which at 650 is low, increases with increasing rise of temperature to a maximum, whereupon the yield diminishes with use of a still higher temperature because of the formation of carbonaceous decomposition products and higher polybenzenes, such as para-diphenyl-benzene. The pyrolysis heretofore has been carried out in various ways, such as by contacting benzene vapors with a heated metallic lament; by passing benzene vapors through an externally heated iron tube; by passing benzene vapors through a molten metal bath, such as molten lead; or by passing benzene vapors through a heated mass of comminuted carbonaceous material. All of such methods possess disadvant-ages, particularly if the diphenyl is to be produced on an industrial scale. For instance, the method involving use of a heated filament leads to considerable tar formation and weakening of the filament because of deposition of carbon thereon. The second method, i. e. passing benzene vapors through an externally heated iron tube, also leads to considerable tar formation and carbonization, and to destruction of the tube due to non-uniform heating and overheating of the latter. The third method, involving passing benzene through a molten metal bath, results in the accumulation of organic by-products, carbonaceous decomposition products, and metal compounds in such bath. 40 The fourth method, i. e. contacting benzene vapors with irregular and discontinuous carbon surfaces, is undesirable in inhibiting a free flow of vapors through the heating zone, thereby resulting in the formation of eddy currents and dead spaces with local overheating and the formation of decomposition products, and, when electrically heated, is undesirable because of unequal electrical resistance with resulting irregular heating of carbon surfaces at the points of 50 contact. Accordingly, all the above mentioned methods are unsatisfactory for the industrial manufacture of diphenyl.

I now have found in heating benzene vapors at substantially atmospheric pressure and at a temperature suflicient to form diphenyl therefrom,

i931. Serial No. M4350 (Cl. E-168) that, (1) if the temperature of the heating surface exceeds about 950 C.,' excessive carbonization and formation of higher polybenzenes occur, even though said vapors are forced past such surfaces at very high speeds, and (2) at surface temperatures below 950 C., as low as 650 C., the slower the speed of said vapors, the greater the tendency to carbonization and formation of higher polybenzenes. Accordingly, both high velocity of the vapors and maintenance of all surfaces, with which said vapors contact, at temperatures below about 950 C. are essential to prevent excessive undesirable reactions and to obtain a high yield of diphenyl. By carrying out my invention as herein described, the heating surfaces are maintained at a temperature of 950 C. or below, thereby preventing the heated gases from contacting with a too highly heated surface and yet supplying the heat of reaction thereto. Any desired speed of vapors through the furnace may be attained by providing for a suitable pressure drop through the system, the speed, of course, not exceeding that at which the vapors may attain a temperature suitablefor the pyrolysis thereof. The higher the speed of said benzene vapors Within the permissible limit, however, the higher conversion thereof into diphenyl.

I further have found that the above said desired operating conditions may be realized by contacting the benzene vapors with a continuous electrically heated conductor, such as rods, plates, or tubes of suitable resistance material, e. g. carbon, silicon carbide, iron-chromium alloy, or equivalent thereof maintained at a temperature not exceeding about 950 C.l by passage of an electric current therethrough.

My invention, then, consists o1' the process and steps hereinafter fully described and particularly pointed out in the claims, the annexed drawing and the following description setting forth in detail certain means and modes of carrying out my invention, such disclosed means and modes illustrating, however, but several of various ways in which the principle of my invention may be used.

In said annexed drawing:-

Fig. 1 represents a vertical tube resistor furnace suitable for converting benzene into diphenyl. Referring to the drawing, the furnace therein illustrated is enclosed by a cylindrical shell 1 provided with a lining 2 of refractory material, and having an outer covering of heat insulating material 3. The upper end of shell 1 is flanged and fitted with a cover 4. A removable tube 5 of chrome-nickel steel, closed at the upper end, is inserted centrally through cover 4, a gas-tight connection being made by means of gland and stuffing ybox 6, the latter being provided with a suitable packing material 7 capable of withstanding high temperature. The lower end of tube 5 communicateswith a carbon tube 8 centrally disposed within shell l and spaced away from lining 2 to form therewith an annular passageway 9, tubes 5 and 8 being connected by graphite connector 10. A lateral opening 11 in tube 5 communicates with the upper end of annular passage 9. The lower end of shell 1 is likewise flanged and closed by a cover 12, but electrically insulated from the latter by gasket 13, the bolts holding cover 12 being also electrically insulated therefrom. A tube 14 is inserted centrally through cover 12 and connects with the lower end4 of carbon tube 8 by means of graphite connector 15. A gas inlet tube 16 is introduced through shell 1 and lining 2 communicating with the lower end of annular passage 9. Electrical ture 11 into tube 5 and downwardly through tube 8 which is heated to a cracking temperature not exceeding about 950 C. by passage of electric current and wherein the temperature of the vapor is raised to the cracking pointy thereof, hydrogen being liberated and diphenyl formed. The pyrolyzed vapors pass ou't of the furnace through lexit pipe 14 which leads to any apparatus suitable for separating the hydrogen, diphenyl and higher-boiling products from the uncracked benzene, and, if desired, returning the latter to the process. y

The electrical conductor materials referred to, i. e. carbon, graphite, or silicon carbide, because of the porosity thereof, are somewhat unsuited for construction of furnaces heated in the usual way. If benzene vapors are passed through heated tubes or chambers constructed thereof, said vapors eventually penetrate such walls and build up carbonaceous deposits on the outer surface, thereby altering the heat capacity -and electrical resistance thereof. This difiiculty may be avoided, however, if both sides or surfaces of the conductor are contacted simultaneously with the vapors to be pyrolyzed, thus substantially equaliz- Ving the pressure on both sides or making it possible to create a negative pressure differential within the reaction zone to prevent the outward diffusion of the vapors.

Rods are less suitable for use as an electrical conductor because of the low ratio of surface to cross section thereof. Accordingly, in carrying out my present invention, I prefer to employ a furnace constructed of tubes or plates, or a multiplicity thereof, of the above said conductor materials, the benzene vapors preferably being contacted with both sides thereof. This has the further advantage of conserving heat energy by obtaining a more efficient heat transfer between the heating element and the benzene vapors. When a carbon conductor tube is used, the benzene may be first vaporized, -passed over the outside of such tube at any desired rate of speed to preheat the same and then passed through the space within the tube, thereby being heated to a temperature sufficient for the splitting of hydrogen therefrom with the ensuing combination of the phenyl radicles to form diphenyl. The vapors within the tube may be maintained atl a lower pressure than the vapors without by properly proportioning the size of the passages, thus avoiding penetration of the heated vapors outwardly through the tube into the preheating zone.

As an example of operating my herein described process, benzene vapors, preheated to a temperature of 550 C., were forced at a rate of 41.3 pounds per hour by a blower into a furnace of the typeshown in Fig. 1, carbon tube 8 being 48 inches in. length and having an internal diameter of inch and an external diameter of 1 inch. The preheated vapors were further heated toa temperature of 750 C. in passage through annular space 9 by contact with carbon tube 8, the outside surface thereof being at a temperature of 870 C. vThe preheated vapors at a temperature of about 750 C., entered tube 5 and then passed downwardly through carbon tube 8, being thereby heated to 780 C., the inside surface of said tube 8 being at a tempertaure of 920 C. The exit vapors, comprising unreacted benzene, diphenyl, higher polybenzenes, and hydrogen, were conducted by means of exit pipe 14 into pipe condensers and coolers (not shown), being therein cooled to 35 C. to condense liquid products therefrom, residual hydrogen being vented. By operating in this manner, approximately 16 per cent of the benzene was pyrolyzed in one pass through the furnace, the crude product comprising about 83 per cent of diphenyl and 17 per cent of higher polybenzenes.

An electrical conductor composed of an ironchromiuin alloy has been found less well adapted for the purpose than the others mentioned for the reason that the electrical resistance thereof may vary during operation of the process because of non-uniformity of structure or carbonization of the metal by decomposition of the vapors, leading to the formation of hot spots and more or less local overheating. This in turn may give rise to severe corrosion of the metal, carbon deposition, and tar formation. By constructing the conductor of carbon, graphite, or silicon carbide, however, such hot spots are avoided and a higher yield of diphenyl obtained without substantial formation of carbon or higher-boiling products.

In general, my improved process for the manufacture of diphenyl comprises passing benzene vapors, preheated by heat exchange with the reacted vapors or other suitable means to a temperature whereat pyrolysis thereof occurs, e. g. about 650 C., through or in contact with an electrical conductor of carbon, graphite, silicon carbide, or equivalent thereof, heated by an electric current to a tempertaure between about 650 and 950 C. The reacted vapors may be cooled by heat exchange with incoming unreacted benzene or its vapors, or by other suitable means, to a temperature whereat unreacted benzene is separated from the diphenyl and higher polybenzene, the so separated and unreacted benzene then being returned to the process and hydrogen vented from the system. Such process may be operated most conveniently at approximately atmospheric pressure, but a higher or lower pressure may be employed, if desired. In brief, some of the advantages of my herein described improved process for the manufacture of diphenyl by the pyrolysis of benzene, are 1) carbon deposition is substantially avoided, 2) formation of secondary decomposition products such as higher poly-phenyl-benzenes is limited to a minimum amount, and 3) diphenyl is obtained in good yield, such results being obtained by the use of an apparatus as herein described whereby the process may be operated industrially; the contact surface being constructed preferably of porous electrically conductive material, and being electrically heated to a temperature between about 650 and 950 C.

In the following claims, the expression carbonaceous conductor shall be understood to refer to an electrical conductor composed of carbon, graphite, graphitized carbon, or any of said Inaterials intermixed with relatively nonconductive materials, e. g. cements or silicon carbide which might be used in making such conductor.

Other modes of applying the principle of my invention may be employed instead of those explained, change being made as regards the steps herein disclosed, provided the details stated by any of the following claims or the equivalent thereof be employed.

I therefore particularly point out and distinctly claim as my invention:-

1. A process of making diphenyl which comprises contacting a current of benzene vapors at a temperature of approximately 650-950 C. with the inner surface of a tubular carbonaceous electrical conductor maintained at substantially the same temperature.

2. A process of making diphenyl which com-- prises preheating benzene vapors on the outside of a carbonaceous tubular electrical conductor to a temperature of approximately 650-850 C., and then heating the preheated vapors on the inside of said conductor, the latter being at a temperature of not over 950 C.

3. A process of making diphenyl which comprises vaporizing benzene, preheating the so obtained vapors on the outside of a tubular carbon electrical conductor to a temperature of cooling the reaction product by heat transfer lwith the benzene vapors to be pyrogenize'd, and

separating crude diphenyl therefrom.

5. A process of making4 diphenyl which comprises vaporizing benzene, preheating the so obtained vapors to a temperature of about '750 C. on the outside of a tubular carbon electrical conductor, then 'further heating the vapors on the inside of said conductor at a temperature not over 950 C., cooling the reaction product by heattransfer with the benzene vapors to be pyrogenized, separating polybenzenes therefrom, condensing benzene from the remaining vapors, venting residual hydrogen, and returning unreacted benzene to the process.

6. In a process of making diphenyl by the pyrolysis of benzene, the steps of preheating benzene vapors by contacting the same with one side of an electrically heated carbonaceous heating element, and then supplying the heat of reaction to such preheated vapors by contacting the latter with the other side of said heating element.

7. In a process of making diphenyl by the pyrolysis of benzene, the steps of preheating benzene vapors by contacting the same with one side of an electrically heated carbon heating element, and then supplying the heat of reaction to such preheated vapors by contacting the `latter with the other side of said carbonaceous heating element.

8. In a process of making diphenyl by contacting benzene vapors with an electrically heated porous carbon heating element, the step of preventing penetration of said benzene vapors therethrough by maintaining benzene vapor at approximately the same pressure on the opposite side thereof.

9. In a process of making diphenyl by contacting benzene vapors with an electrically heated carbon conductor, the step of heating said benzene vapors on both of opposite sides of said carbon conductor to prevent penetration of benzene vapor therethrough and variation of the electrical resistance thereof by carbonization of said penetrated vapors.

10. A method of making diphenyl which comprises conducting a current of vaporized benzene into contact with one surface of an electrically heated carbonaceous conductor whereby the benzene is preheated almost to its dehydrogenation temperature and then contacting the current of preheated benzene vapors with the opposite surface of the same carbonaceous conductor whereby the benzene is heated to its dehydrogenation 

